Oversimplification:
If u wanted to invest, you had the choice between USA or Japan, the Dollard, or the Yen. The yen went down the drain so the dollard gained the monopole. Great for USA, not good for the rest of the world.
EURO was created to balance the Dollard (one of the reason), and it kind of works as the EURO keeps up with the Dollard. Now you can invest in USA, or in Europe.